An anti-sway or anti-roll bar is intended to force each side of the vehicle to lower, or rise, to similar heights, to reduce the sideways tilting (roll) of the vehicle on curves, sharp corners, or large bumps.
Although there are many variations in design, a common function is to force the opposite wheel's shock absorber, spring or suspension rod to lower, or rise, to a similar level as the other wheel. In a fast turn, a vehicle tends to drop closer onto the outer wheels, and the anti-sway bar soon forces the opposite wheel to also get closer to the vehicle. As a result, the vehicle tends to “hug” the road closer in a fast turn, where all wheels are closer to the body. After the fast turn, then the downward pressure is reduced, and the paired wheels can return to their normal height against the vehicle, kept at similar levels by the connecting anti-sway bar.
A negative side-effect of connecting pairs of wheels is that a jarring or bump to one wheel tends to also jar the opposite wheel, causing a larger impact applied across the whole width of the vehicle. A vehicle that runs over several potholes scattered in the road tends to rock side-to-side, or waddle, due to the action of the bar at each pair of wheels. Other suspension techniques can delay or dampen this effect of the connecting bar, as when hitting small holes that momentarily jolt a single wheel, whereas larger holes or longer tilting then tugs the bar with the opposite wheel.
A conventional anti-sway bar is usually a torsion spring that resists body roll motions. It is usually constructed out of a cylindrical steel bar, formed into a “U” shape that connects to the body at two points, and at the left and right sides of the suspension. If the left and right wheels move together, the bar rotates about its mounting points. If the wheels move relative to each other, the bar is subjected to torsion and forced to twist. Each end of the bar is connected to an end link through a flexible joint. The anti-sway bar end link connects in turn to a spot near a wheel or axle, transferring forces from a heavily-loaded axle to the opposite side.
The bar resists the torsion through its stiffness. The stiffness of an anti-roll bar is proportional to the stiffness of the material, the fourth power of its radius, and the inverse of the length of the lever arms (i.e., the shorter the lever arm, the stiffer the bar). Stiffness is also related to the geometry of the mounting points and the rigidity of the bar's mounting points. The stiffer the bar, the more force required to move the left and right wheels relative to each other. This increases the amount of force required to make the body roll.
Excessive roll stiffness, typically achieved by configuring an anti-roll bar too aggressively, can make the inside wheels lift off the ground during hard cornering. This can be used to advantage: many front wheel drive production cars lift a rear wheel when cornering hard in order to overload the opposite wheel, limiting understeer.
There are active anti-sway systems that are controlled by a controller provided linked to the suspension ECU or body sensors and electric motors. There are even systems that can be used to disengage the stabilizer bars when off-road, allowing for greater vehicle articulation and ride quality. Most prior art system include an elaborate array of linkages, cables and levers or complex components that are prone to the elements and to damage by rocks, mud, sand, being hit by sticks and so on.
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